Preparing Metal for Machining / Etching

Cleaning the Metal

The selected metal has to be cleaned prior to having the photo resist applied (refer to the process flowchart). The cleaning operation is necessary to remove the oil, grease or any substance from the surface of the metal that would prevent good adhesion of the photo resist. There are 2 methods of cleaning, mechanical and chemical. Mechanical cleaning usually involves some form of scrubbing in conjunction with a suitable mild degreasing solution. This method gives a good result but is not practical for very thin gauge metals. For example 0.013mm stainless steel or 0.050mm copper is almost impossible to scrub without causing mechanical damage. Experiments have been conducted with ultrasonic cleaning; it has proved to be a slow process that is probably not best suited for this type of application. Chemical cleaning is a mild pickling process. The sheets of raw material are suspended in a degreasing solution that typically consists of 30% phosphoric acid plus other degreasing agents as added by the solution manufacturer. The proprietary cleaner is diluted to a 10% solution but this is of course dependent on the manufacturer’s recommendations. Some heating of the solution is beneficial. The temperature is not critical; typically 45 degrees centigrade gives a good clean. After soaking for approximately 10 minutes the sheets are given a clean water rinse. The next process is dependant on the type of photo resist to be applied (refer to Laminating).

Photo Resist Protective Coating Types

There are 4 types of photo resist - wet film positive, wet film negative, dry film positive and dry film negative. Negative film requires exposure to UV light to harden the film whereas positive film is softened when exposed to UV light. Positive film is commonly used in the printed circuit industry but offers no advantage to the component manufacturing industry. Italix uses a negative dry film for producing all parts.

Image Creation: Printing & Developing the Image

PrintingThe printing of the metal sheet is carried out by using a punch registered envelope which consists of 2 photographic films (refer to Production of Phototools) and an ultra violet light source. The object of the printing and developing process is to produce sheets that have cut lines i.e. bare metal areas on both faces where the etchant is required to act. Where the metal is to be unaffected by the process it remains covered in photo resist film.

Registration of the films to each other is critical to achieve component
accuracy as is flatness of raw material (refer to raw materials). A bowed
sheet will cause progressive misalignment or out of registration image.

Developing
Developing is the term used for the chemical removal of the unplasticised areas of the photo resist. The printed sheets are passed through the developing machine on a conveyor, where the unexposed areas of resist are removed with a sodium carbonate based solution. It is during the developing stage the bare metal cutting lines are produced. The developing operation is the final task before the sheets are subjected to the photo chemical machining process. If dry film resist is used the polymer film is removed prior to developing. Once developed the prepared sheets are no longer sensitive to ultra violet light and can be freely removed from the controlled light spectrum of the print room area. Special care has to be taken when very fine lines (less than 0.15mm wide) are present on the phototool. It is often necessary to double develop that is passing the exposed sheet through the developer twice. A small piece of undeveloped photo resist present on the cut line will always result in an unsatisfactory etch of that specific component. Even the smallest piece of photo resist present on a cutting line will prevent the etchant attacking that area. If for processing reasons the material sheets have to be double laminated, then double developing is always a necessity.

ManufactureOne of the key strengths of the etching process is that the chemistry is unaware of the hardness (or type) of material that it is etching through. This is due to the way that etchant chemistry breaks down the materials grain structure, which only changes slightly with temper (refer to materials section). The component etching process was originally developed in the 1940s to manufacture parts from materials too hard to stamp or machine. Etching also has no detrimental effect on the properties of the material surrounding the etched areas. Concerns of localized annealing or embitterment do not occur during the process. This gives etching the advantage of being able to create component shapes in any hardness of material from annealed to spring hard. The ranges of material types/thickness regularly etched in commercial production are listed in the Raw Materials section.

EquipmentThe modern spray-etching machine is almost universally used in production photo chemical machining. The work piece, or sheet, travels along a horizontal conveyor consisting of rubber wheels on GRP rods which carries it through a rigid PVC chamber, where it is vigorously sprayed with hot etchant from batteries of nozzles above and below the track. The most productive etch rate is achieved when the etchant is sprayed perpendicular to the work piece. This ensures that as the cut moves through the work piece the main pressure is directed to the base of the cut, therefore only attacking the side walls by diffusion.

Edge ProfileThe cutting action of the chemicals does create a characteristic edge profile referred to as a bi-cuspic edge. This has the appearance of a seagull in flight. This is the result of simultaneous etching from both sides leaving a witness at the point of breakthrough. As the etching proceeds the bicuspid edge retreats at a decreasing rate. The nearer to the horizontal the surface the faster it etches, therefore the protruding cusp is reduced faster than the nearly vertical sidewalls.

As etching continues the bicuspid edge becomes progressively straighter and almost vertical. Further etch back would result in over etching and a concave edge protruding into the materials section by as much as 1/5th of the materials thickness. The phototool will have been sized to achieve nominal dimension at straight wall (refer to phototool production). The rate of etch slows as the edge profile becomes vertical. Consequently if the product comes to size midway through a machine pass it will not be over etched and therefore undersize by the end of the pass.

ChemistryThe majority of photo chemical machining is carried out with aqueous solutions of ferric chloride. It is inexpensive, readily available, versatile in that it attacks the majority of commonly used engineering metals and alloys, and it has a high capacity for dissolving those metals. Environmentally it is attractive as it is of low toxicity and relatively easy to filter, replenish and recycle. Also it is used extensively in water treatment processes. Ferric nitrate is used for the etching of non-standard materials such as molybdenum and silver. Ferric chloride is a black solid that dissolves in water to give a clear, red brown, astringent solution. Ferric is supplied in 2 principle grades, or purchased in one condition and treated on site to suit application. Due to the importance of chemistry in the etching process it is common to have onsite technical support in the form of laboratory facilities plus chemistry technicians. This allows for regular monitoring and adjustment of the condition of the production chemistry. Regular monitoring is essential, as the properties of the etchant chemistry will alter constantly as they react to the elements transferred into solution from the materials etched. The elements added to the chemistry can increase or decrease the effective strength of the etchant. A consistent etch rate is important for production rate, etch ...and dimensional stability of the processed components, particularly over a large batch.

Multi Stage EtchingA key attribute of photo chemical machining is its flexibility as demonstrated during a multi stage etching procedure which relies upon repetition of elements of the manufacturing process. For example: Take a component design requiring partial thickness reduction in a small area to say 3/4 of material thickness (t). Half etching from one side during the main profile cut would result in a finish thickness of 1/2 (t) in that area, this would be too deep. The only way to achieve a 3/4 (t) would be to selectively etch the area requiring the thickness reduction initially.

The sheet of material would be prepared, laminated and printed with a single sided phototool exposing the area to be surfaced etched only (the component profile would remain undeveloped at this stage). The sheet would be etched until the correct thickness had been achieved in the exposed area. Once complete, the sheet is stripped of resist, cleaned and prepared for re-laminating and printing. This time a double-sided phototool is used which includes the component profile and any through feature details. The previously surface etched area is not developed this time. The sheet is then etched until the components are to finished size; the surface etched area is not attacked by the etchant during this second stage due to a protective tent of new photo resist. Once the component has completed its second etching stage the resist is stripped off as normal and the finished part, complete with its accurately thinned area is ready for any further manufacturing stage.

Removal of Resist (stripping)
Photo resist can only be removed by chemical means to avoid damage to the etched components. Different chemicals are utilized to strip different resist formulations. However the photo resist in most common usage is aqueous dry film that can be effectively stripped using a mild caustic soda solution. The stripper solution is applied onto sheets of components in a conveyorised machine; this also has rinsing and drying chambers. Alternatively if the components are loose, then they will be hand stripped in mesh baskets in dip tanks. Once stripped, the components are dimensionally inspected and ready for any appropriate secondary operations, such as plating forming, machining, assembly etc. or shipment to the customer if no additional processes are required.

Economic of the Photo Chemical Machining Process (PCM)

PCM is specified where the required component material is: too hard, too soft, too thin or too complex for conventional metalworking techniques and where freedom from burrs and/or stress is paramount. If the complexity of the profile makes press tooling expensive or impractical or if the quantity of parts required is too small to justify the cost of press tooling. The unit cost of a photo chemically machined part is higher than that of a pressed product because consumables and the overheads of this high technology manufacturing operation are relatively high, (e.g. you double the cost of the raw material when you coat it in resist). However, the tooling costs are low typically $275.00 – $375.00. But most importantly the lead-time is measured in hours. A press shop can produce parts at a high rate and cheaply but the tooling cost is high and the lead-time is weeks, if not months.

However the decision upon method of production should not be made solely on this basis since one or more post blanking operations may be avoided by the use of the etching process. Take the example of full hard spring steel products. Traditional methods would require the stamping of the components from annealed material then hardening by heat treatment. This carries with it a risk of distortion and burrs, which may require either a design compromise or additional finishing operations. If the components were to be photo chemically machined, they could be manufactured from full hard material thereby avoiding any risk of distortion, burrs and subsequent finishing costs.